Actually it does, since Fireball does half damage on a successful Dex save. Half cover gives +2 to AC and Dex saves, 3/4 cover gives +5 to AC and Dex saves. Granted, cover won’t save a creature from taking damage from Fireball, but it will give them a better chance of reducing the damage by half, which is better than nothing.
Thing is that fireball says it goes around corners, so cover doesn't really "do anything" in this case. Its not blocking the fireball at all.